1. Field of the Invention
The present invention relates generally to dendritic cells and their role in T cell activation. More specifically, the invention relates to the identification and isolation of genes encoding dendritic cell surface proteins required for T cell activation. The invention further relates to the production and use of reagents that inhibit or augment dendritic cell-dependent activation of T cells.
2. Description of Related Art
Dendritic cells (DC) are unique among antigen presenting cells (APC) by virtue of their potent capacity to activate immunologically naive T cells (Steinman, 1991). DC express constitutively, or after maturation, several molecules that mediate physical interaction with and deliver activation signals to responding T cells. These include class I and class II MHC molecules. CDSO (B7-1) and CD86 (B7-2). CD40. CD11a/CD18 (LFA-1), and CD54 (ICAM-1) (Steinman, 1991; Steinman et al., 1995). DC also secrete, upon stimulation, several T cell-stimulatory cytokines, including IL-1.beta., IL-6. IL-8, macrophage-inflammatory protein-1.alpha. (MIP-1.alpha.) and MIP-1.delta. (Matsue et al., 1992; Kitajima et al., 1995; Ariizumi et al., 1995; Caux et al., 1994; Heufler et al., 1992; Schreiber et al., 1992; Enk et al., 1992; Mohamadzadeh et al., 1996). Both of these properties, adhesion molecule expression and cytokine production are shared by other APC (e.g., activated macrophages and B cells), which are substantially less competent in activating naive T cells.
T cell activation is an important step in the protective immunity against pathogenic microorganisms (e.g., viruses, bacteria, and parasites), foreign proteins, and harmful chemicals in the environment. T cells express receptors on their surface (i.e., T cell receptors) which recognize antigens presented on the surface of antigen-presenting cells. During a normal immune response, binding of these antigens to the T cell receptor initiates intracellular changes leading to T cell activation. DC express several different adhesion (and costimulatory) molecules, which mediate their interaction with T cells. The combinations of receptors (on DC) and counter-receptors (on T cells) that are known to play this role include: a) class I MHC and CD8, b) class II MHC and CD4, c) CD54 (ICAM-1) and CD11a/CD18 (LFA-1), d) ICAM-3 and CD11a/CD18, e) LFA-3 and CD2, f) CD80 (B7-1) and CD28 (and CTLA4), g) CD86 (B7-2) and CD28 (and CTLA4) and h) CD40 and CD40L (Steinman et al., 1995). Importantly, not only does ligation of these molecules promote physical binding between DC and T cells, it also transduces activation signals.
C-type lectins are a family of glycoproteins that exhibit amino acid sequence similarities in their carbohydrate recognition domains (CRD) and that bind to selected carbohydrates in a Ca.sup.2+ -dependent manner. C-type lectins have been subdivided into four categories (Vasta et al., 1994; Spiess 1990). The first group comprises type II membrane-integrated proteins, such as asialoglycoprotein receptors, macrophage galactose and N-acetyl glucosamine (GlcNac)-specific lectin, and CD23 (Fc.epsilon.RII). Many members in this group exhibit specificity for galactose/fucose, galactosarnine/GalNac. or GlcNac residues. The second group includes cartilage and fibroblast proteoglycan core proteins. The third group includes the so-called "collectins" such as serum mannose-binding proteins, pulmonary surfactant protein SP-A, and conglutinin. The fourth group includes certain adhesion molecules which are known as LEC-CAMs (e.g., Mel-14, GMP-140, and ELAM-1).
C-type lectins are known to function as agglutinins, opsonins, complement activators, and cell-associated recognition molecules (Vasta et al., 1994; Spiess 1990; Kery, 1991). For instance, macrophage mannose receptors serve a scavenger function (Shepherd et al., 1990), as well as mediating the uptake of pathogenic organisms, including Pneumocystis carinii (Ezekowitz et al., 1991) and Candida albicans (Ezekowitz et al., 1990). Serum mannose-binding protein mimics Clq in its capacity to activate complement through the classical pathway. Importantly, genetic mutations in this lectin predispose for severe recurrent infections, diarrhea, and failure to thrive (Reid et al., 1994). Thus, C-type lectins exhibit diverse functions with biological significance.
Importantly, carbohydrate moieties do not necessarily serve as "natural" ligands for C-type lectins. For example, CD23 (FC.epsilon.RII), which belongs to the C-type lectin family as verified by its binding of Gal-Gal-Nac (Kijimoto-Ochiai et al., 1994) and by its CRD sequence, is now known to recognize IgE in a carbohydrate-independent manner; an enzymatically deglycosylated form of IgE as well as recombinant (non-glycosylated) IgE produced in E. coli both bind to CD23 (Vercelli et al., 1989). Thus, some C-type lectins recognize polypeptide sequences in their natural ligands. Even more extreme is the recent hypothesis that a major biological finction of lectins is the recognition of polypeptides, instead of carbohydrates, as suggested by the identification, from a random polypeptide library, of several polypeptide ligands for Con A, a prototypic plant lectin (Oldenburg et al., 1992).
Recently, two C-type lectins have been identified on DC surfaces. First, Jiang et al. cloned the protein recognized by the NLDC-145 mAb, one of the most widely used mAb against murine DC (Jiang et al., 1995). This protein, now termed DEC-205, was found to be a new member of the C-type lectin family, one that contains ten distinct CRD. Second, Sallusto et al reported that human DC express macrophage mannose receptors (MMR), which also contain multiple CRD (Sallusto et al., 1995). Both receptors have been proposed to mediate endocytosis of glycosylated molecules by DC, based on the observations that: a) polyclonal rabbit antibodies against DEC-205 not only bound to DEC-205 on DC surfaces, but were subsequently internalized; b) these DC activated effectively a T cell line reactive to rabbit IgG; and c) internalization of FITC-dextran by DC was blocked effectively with mannan, a mannose receptor competitor (Jiang et al., 1995; Sallusto et al., 1995). With respect to cell type specificity, DEC-205 is now known to be also expressed, albeit at lower levels, by B cells and epithelial cells in thymus, intestine.,and lung (Witmer-Pack et al., 1995; Inaba et al., 1995) and MMR is also expressed even more abundantly by macrophages (Stahl 1992). Thus, there have been no C-type lectins that are expressed in a DC-specific manner.
Since it is known that DC are far more potent than other APC in their capacity to activate immunologically naive T cells, it is probable that DC express a protein or proteins which function to activate T cells and which are not expressed by other APC. Knowledge of the structure of such a protein or proteins would prove quite valuable in many areas. For example, the purified protein(s) could be used to identify its (or their) ligands expressed by T cells. Additionally, antibodies could be raised against the purified protein(s) and used to inhibit DC-mediated T cell activation.